The present invention relates to the fuel-bearing "seed" core of a fast breeder reactor, and in particular to the specific composition and structure of fuel pellets for use in the core.
Breeder reactors produce energy from fission reactions in fissionable material. At the same time, excess neutrons that are not used to maintain the energy-releasing nuclear reaction are used to transmute fertile material into fissionable material. For instance, fertile uranium 238 atoms that absorb neutrons produced by a plutonium fission could ultimately be transformed by the neutron captures into fissionable plutonium atoms. Accordingly, the fuel pellets of the seed region are typically made of fissionable plutonium mixed with fertile uranium. Since the power output of each fuel pellet is determined to a large extent by the amount of fissionable material in the pellet, and since the ability of the pellet to transfer its heat to a coolant is dependent upon its surface area, it is necessary to dilute the fissionable plutonium with fertile uranium so that the individual pellets will have a high enough mass-to-surface-area ratio to make them a manageable size. In order to strike a balance between maintaining a relatively long refueling interval and keeping the fuel inventory from becoming too large, this dilution typically results in a ratio of fertile atoms to fissile atoms of between about 4:1 and about 9:1.
It is important in any heat-transfer system that the temperature difference between the heating medium and the cooling medium be high enough to effect an efficient transfer of energy. As a result, it is desirable for the fuel in a reactor to be relatively hot. However, uranium and, especially, plutonium have fairly low melting points, so they are not used in their pure form. They are chemically combined with oxygen, nitrogen, silicon, or carbon, for instance, to convert them into ceramics that have high resistances to heat. In order to achieve a neutron efficiency high enough to obtain a breeding gain without building a prohibitively large core, the fuel should be made of a ceramic that is as dense as possible. For this reason carbide fuels have been of interest, because they are more dense than the oxide fuels typically used. If the density of the carbide fuels can be used to advantage, a breeding gain will be achieved with a cofre of smaller size than would be required for an oxide core. It is a feature of the carbide fuels, however, that they tend to be subject to swelling. This is a problem because the ceramic pellets are loaded into rods made of a thin metal cladding, which could easily be pierced by an expanding fuel pellet. Accordingly, carbide fuels make it necessary that a larger gap be left in the rod to permit the pellets to expand. This gap detracts from the overall density of the core, so some of the advantage gained by the increased density of the carbide fuel is lost by the provision of extra space in the fuel elements. In addition, a larger gap increases the probability that particles of cracked pellets will lodge between the pellets and the clad, thereby tending to cause failed fuel.
Since it has been determined that a significant portion of the swelling in carbide-fuel pellets is caused by gaseous fission products that are trapped within the pellets, it has been proposed to make the pellets of porous material, thereby giving the gaseous fission products a means of escaping the pellet. Again, however the solution has a density penalty that is higher than is desirable.